1. Field of the Invention
The present invention relates in general to improvements in a fluorescent display device and in a method of manufacturing the fluorescent display device.
2. Discussion of the Related Art
There is known a fluorescent display tube which includes: (a) a substrate having a display surface; (b) anodes formed on the display surface of the substrate and spaced apart each other; (c) fluorescent layers each of which is fixed to a corresponding one of the anodes; (d) filament cathodes located above the fluorescent layers to generate thermo electrons; and (e) a control electrode (grid electrode) located between the fluorescent layers and the cathodes, and consisting of a plurality of sections spaced apart each other. The control electrode serves to control activations of the fluorescent layers, such that the fluorescent layers are selectively activated, namely, emit glow or light when they are struck by the thermo electrons in a vacuum space. This type of fluorescent display tube is capable of providing a clear image with a relatively low voltage to accelerate the electrons, owing to the arrangement in which the fluorescent layers are positioned in the vicinity of the cathodes which generate the electrons toward the fluorescent layers. Further, the use of different fluorescent materials for the fluorescent layers which emit lights of different colors permits a color display of images. Therefore, the fluorescent display tube is widely used as display devices on acoustic devices and on instrument panels of motor vehicles or airplanes. Particularly, in such a fluorescent display tube having, in place of a so-called “mesh-grid structure” in which the grid electrodes are provided by meshes covering the fluorescent layers, a so-called “rib-grid structure” in which the grid electrodes are provided by conductive films fixed to upper end face of a partition or rib that surrounds the fluorescent layers and that has a larger height than the fluorescent layers, the grid electrodes are unlikely to suffer from thermal deformations even where the size of each grid electrode is increased in the interest of increasing the overall size or area of the display screen, thereby making it possible to prevent a problem such as instable luminance of the fluorescent layers and short-circuiting which would be caused by the thermal deformations of the grid electrodes. Further, while the luminance of the fluorescent layers is problematically reduced depending upon an opening ratio of the meshes in the fluorescent display tube having the mesh-grid structure, such a problem no longer exists in the fluorescent display tube having the rib-grid structure in place of the mesh-grid structure.
The fluorescent display tube having the above-described rib-grid structure is generally produced in the following manner:
That is, the rib is formed in such a manner that permits the rib to surround the anodes which have been fixed to the display surface of the substrate. After the formation of the rib, the fluorescent layers are fixedly formed on the anodes in accordance with a suitable method such as a thick-film screen printing method in which a fluorescent paste is dropped into recesses or cells defined by the rib. After the formation of the fluorescence layers, the grid electrode is fixedly formed on the upper end face of the rib in accordance with a suitable method such as a thick-film screen printing method in which a conductor paste is applied to the upper end surface of the rib. In this instance, it is desirable to minimize the width of each wall of the rib, for minimizing the surface area of a non-display portion of the display screen, to such an extent that still permits the grid electrode to be formed on the rib. In view of this, the rib and the grid electrode are formed by using the same screen printing pattern, for equalizing the width of the rib and the width of the grid electrode to each other.
Where such a fluorescent display tube is designed for graphical representations, the plurality of fluorescent layers are arranged in the longitudinal and width directions of the substrate with high density, for forming a desired image in a matrix of dots. It is desirable that the spacing interval between each adjacent pair of the fluorescent layers is minimized for thereby improving the quality of the formed image. Therefore, between each adjacent pair of the fluorescent layers, there is provided only a single grid electrode for serving commonly for both of the adjacent pair of the fluorescent layers. However, in such a arrangement, each fluorescent layer can not be surrounded at its entire periphery by the grid electrode, namely, a certain amount of gap has to be provided in the grid electrode so that the grid electrode is constituted by a plurality of sections which are spaced apart each other with the certain amount of gap between each adjacent pair of the plurality of sections. As long as the rib and the grid electrode are formed by using the same screen printing pattern, the provision of the gap in the grid electrode leads to the provision of a gap 82 in the rib 80, as shown in FIGS. 1A and 1B. As a result, the fluorescent paste 84 dropped into each square cell defined by the rib 80 tends to flow out of the square cell, as indicated by the arrows in FIG. 1A, through the gap 82, and then brought into contact with the anode 86 or fluorescent layer 12 located in the adjacent cell, causing problematic short-circuiting between the anodes 86 adjacent to each other, or between the anode 86 and the fluorescent layer 88 adjacent to each other. Namely, the conventional fluorescent display tube suffers from a risk of short-circuiting between segments located in the respective cells which are adjacent to each other. It is noted that the term “segment” may be interpreted to mean either of the anode and fluorescent layer in the following description.